1. Field of the Invention
The present invention relates to a thin-film transistor substrate, a liquid crystal display unit provided with the same and a manufacturing method of the thin-film transistor substrate. More particularly, the invention relates to a technique capable of improving transistor properties by using a special structure for the connecting portion between source and drain electrodes and a semiconductor active film.
2. Description of the Related Art
FIGS. 6 and 7 illustrate a typical structure of a thin-film transistor array substrate provided with various parts such as a gate wiring G and a source wiring S on a substrate in a conventional thin-film transistor type liquid crystal display unit. In the thin-film transistor array substrate shown in FIGS. 6 and 7, the gate wiring G and the source wiring S are arranged in a matrix shape on a transparent substrate 6 such as a glass one. The region surrounded by the gate wiring G and the source wiring S serves as a picture element section 1 in which a thin-film transistor 3 is provided.
The thin-film transistor 3 shown in FIGS. 6 and 7 has a general configuration of the etch-stopper type, and comprises a gate insulating film 9 provided on a gate wiring G and an gate electrode 8 provided by extracting from the gate wiring G; a semiconductor active film 10 comprising amorphous silicon (a-Si) provided on this gate insulating film 9 oppositely to the gate electrode 8; and a drain electrode 11 and a source electrode 12 comprising a conductive material provided opposite to each other on this semiconductor active film 10. On the upper sides of the both sides of the semiconductor active film 10, there are formed ohmic contact films 10a and 10a made of amorphous silicon by doping impurities serving as donors such as phosphorus at high concentrations, and an etching stopper 13 held between the drain electrode 11 and the source electrode 12 is formed thereon. A transparent picture element electrode 15 comprising a transparent electrode material is connected so as to range from above the drain electrode 11 to a side of the drain electrode 11.
A passivation film 16 is provided so as to cover the gate insulating film 9, the transparent picture element electrode 15, the drain electrode 11 and the source electrode 12. An orientation film not shown is formed on the passivation film 16. An active matrix liquid crystal display unit is formed by providing a liquid crystal above this orientation film, so that impression of an electric field to liquid crystal molecules by means of the transparent picture element electrode 15 permits orientation control of the liquid crystal molecules.
The liquid crystal display unit of the above-mentioned construction has a configuration in which a back light is provided on the back of the transparent substrate 6, and the user can recognize bright or dark from whether the orientation-controlled liquid crystal interrupts or allows to transmit a light emitted from the back light.
However, when a part of the light entering the transparent substrate 6 should reach the semiconductor active film 10 between the drain electrode 11 and the source electrode 12, a charge is produced in the semiconductor active film 10 through excitation by the light, causing photocurrent to flow. When driving the thin-film transistor, therefore, leakage current would flow although the circuit is turned off. The flow of such leakage current causes an increase in turnoff current (I.sub.OFF) during driving of the liquid crystal, and this may adversely affect light transmissivity of the liquid crystal.
For the purpose of avoiding this inconvenience, there is proposed a structure in which the gate electrode 8 is formed of a light shielding conductive film by forming the gate electrode 8 into a size larger than the semiconductor active film 10 so as to prevent the light of the back light from reaching the semiconductor active film 10.
FIG. 8 illustrates a typical thin-film transistor structure of this kind: the thin-film transistor 27 comprises a gate electrode 21 made of a light-shielding conductive material provided on a substrate 20; a gate insulating film 22 covering the gate electrode 21; a semiconductor active film 23 smaller than the gate electrode 21 provided opposite to the gate electrode 21 on the gate insulating film 22; ohmic contact films 24 and 24 provided on the both sides of the semiconductor active film 23; a source electrode 25 covering one of the ohmic contact films 24; and a drain electrode 26 covering the other ohmic contact film 24.
With the structure shown in FIG. 8, in which the gate electrode 21 serves also as a light shielding layer, it is possible to prevent the light from the back light from entering the semiconductor active film 23, and ensure satisfactory electrical contact of the source electrode 25 and the drain electrode 26 with the semiconductor active film 23 under the effect of the ohmic contact films 24 and 24.
With the structure shown in FIG. 8, however, measurement of OFF-current (I.sub.OFF) and ON-current (I.sub.ON) as a thin-film transistor gives a curve as shown in FIG. 10, suggesting a problem of impossibility to achieve a sufficiently low OFF-current. As a result of search for causes carried out by the present inventors, this is attributable to the fact that, in the structure shown in FIG. 8, an end of the semiconductor active film 23 imparted a strong electric field facing the source electrode 25 is in direct contact with the source electrode 25, or an end of the semiconductor active film 23 facing the drain electrode 26 is in direct contact with the drain electrode 26, at portion e in FIG. 8, and this makes it impossible to obtain a sufficient Hall blocking effect.
FIG. 9 illustrates another example of conventional structure of thin-film transistor. The thin-film transistor 28 in this example comprises an ohmic contact film 29 provided so as to cover the end of the semiconductor active film 23 and the gate insulating film 22 on the side thereof, i.e., so as to be laminated over the source electrode 25 at the bottom of the source electrode 25, and another ohmic contact film 29 provided so as to be laminated over the drain electrode 26 at the bottom of the drain electrode 26.
In the structure shown in FIG. 9, however, measurement of OFF-current (I.sub.OFF) and ON-current (I.sub.ON) as a thin-film transistor results in a curve b as shown in FIG. 10: while the value of OFF-current can be reduced sufficiently, the value of ON-current cannot be increased.
When manufacturing the thin-film transistor 28 of the structure shown in FIG. 9, the semiconductor active film 23 is once formed on the entire upper surface of the gate insulating film 22 to form the semiconductor active film 23 on the gate insulating film 22. When patterning this film to achieve the semiconductor active film 23 having an island shape with a target size, the upper surface of the semiconductor active film 23 is susceptible to easy contamination, and even formation of the ohmic contact films 29 thereafter cannot ensure sufficient electric connection between the semiconductor active film 23 and the ohmic contact films 29. This is considered to be a cause of the above inconvenience.